The present invention relates to a micromachine switch used in a milliwave band to microwave band.
Switch devices such as a PIN diode switch, HEMT switch, micromachine switch, and the like are used in a milliwave band to microwave band. Of these switches, the micromachine switch is characterized in that the loss is smaller than that of the other devices, and a compact high-integrated switch can be easily realized.
FIG. 21 is a perspective view showing the structure of a conventional micromachine switch. FIG. 22 is a plan view of the micromachine switch shown in FIG. 21.
A micromachine switch 101 is constructed by a switch movable element 111, support means 105, and switch electrode 104. The micromachine switch 101 is formed on a dielectric substrate 102 together with two RF microstrip lines 121a and 121b. A GND plate 103 is disposed on the lower surface of the dielectric substrate 102.
The microstrip lines 121a and 121b are closely disposed apart from each other at a gap G. The switch electrode 104 is disposed between the microstrip lines 121a and 121b on the dielectric substrate 102. The switch electrode 104 is formed to have a height lower than that of each of the microstrip lines 121a and 121b. 
The switch movable element 111 is arranged above the switch electrode 104. A capacitor structure is formed by the switch electrode 104 and switch movable element 111.
As shown in FIG. 22, since a length L of the switch movable element 111 is larger than the gap G, two ends of the switch movable element 111 oppose the end portions of the microstrip lines 121a and 121b, respectively. The switch movable element 111 is formed to have a width g equal to a width W of each of the microstrip lines 121a and 121b. 
The switch movable element 111 is cantilevered on the support means 105 fixed on the dielectric substrate 102.
As shown in FIG. 21, the switch movable element 111 is generally arranged above the microstrip lines 121a and 121b. With this structure, since the switch movable element 111 is not in contact with the microstrip lines 121a and 121b, the micromachine switch 101 is in an OFF state. At this time, a little high-frequency energy is transmitted from the microstrip line 121a to the microstrip line 121b. 
When, however, a control voltage is applied to the switch electrode 104, the switch movable element 111 is pulled down by an electrostatic force. When the switch movable element 111 is brought into contact with the microstrip lines 121a and 121b, the switch movable element 111 is set in an ON state. At this time, the high-frequency energy from the microstrip line 121a is transmitted to the microstrip line 121b through the switch movable element 111.
As described above, the two ends of the switch movable element 111 oppose the microstrip lines 121a and 121b, respectively. Accordingly, the capacitor structures are also formed between the switch movable element 111 and the microstrip lines 121a and 121b. 
This makes the capacitive coupling between the switch movable element 111 and microstrip lines 121a and 121b so that the high-frequency energy from the microstrip line 121a leaks out into the microstrip line 121b even if the micromachine switch 101 is in the OFF state. That is, in the conventional micromachine switch 101, an OFF isolation characteristic is poor.
A capacitance between the switch movable element 111 and the microstrip lines 121a and 121b is proportional to the opposing area between them. Accordingly, an increase in opposing area increases energy leakage, thereby degrading the isolation characteristic. On the contrary, a decrease in opposing area may improve the isolation characteristic. Therefore, the isolation characteristic can be improved by decreasing the width g of the switch movable element 111.
However, a high-frequency characteristic impedance of a line is related to the surface area of the line, and a decrease in width of the line increases the characteristic impedance. Thus, if the width g of the switch movable element 111 decreases, the characteristic impedance on the gap G increases in the ON state of the micromachine switch 111.
High-frequency energy reflection occurs at a discontinuous portion in the line. An increase in characteristic impedance on the gap G results in impedance mismatching. Thus, since the reflection increases in the ON state of the micromachine switch 101, the ON reflection characteristics degrades.
For example, the microwave switching circuit requires the isolation characteristic of approximately 15 dB or more and the reflection characteristics of approximately xe2x88x9220 dB or less.
The present invention has been made to solve the above problem, and has as its object to suppress the degradation of the ON reflection characteristic of the micromachine switch and improve the OFF isolation characteristic.
In order to achieve the above object, the present invention comprises at least two distributed constant lines disposed close to each other, a movable element arranged above the distributed constant lines such that distal end portions of the movable element oppose the distributed constant lines, respectively, and connecting the distributed constant lines to each other in a high-frequency manner upon contacting the distributed constant lines, and driving means for displacing the movable element by an electrostatic force to bring the movable element into contact with the distributed constant lines, wherein the movable element has at least two projections formed by notching an overlap portion of the movable element which is located on at least one distributed constant line side, and the projections oppose a corresponding distributed constant line. This decreases the opposing area between the movable element and the distributed constant line, thereby reducing the capacitive coupling of the movable element and distributed constant line without decreasing the width of the movable element. When the projection has a width (the length in the direction parallel to the widthwise direction of the distributed constant lines) 1/n (where n is a real number larger than 1) the width of the movable element main body (a portion of the movable element except for the projections), the projection has a high-frequency characteristic impedance much lower than n times the characteristic impedance of the movable element main body. On the other hand, the characteristic impedance of an end portion of the movable element is the synthetic impedance of the projections formed in parallel. Therefore, even the end portion of the movable element can obtain the characteristic impedance almost equal to that of the movable element main body, thereby suppressing the degradation of an ON reflection characteristic of the micromachine switch and improving an OFF isolation characteristic.
In the present invention, movable element main body serving as a portion of the movable element except for projections has a width serving as a length in a direction parallel to the widthwise direction of the distributed constant lines to be equal to a width of each of the distributed constant lines, and, a portion of the overlap portion of the movable element except for two ends in the movable element is notched. With this structure, the characteristic impedance on a gap becomes almost equal to that of each of the distributed constant lines. Thus, the degradation of an ON reflection characteristic of the micromachine switch can be prevented and an OFF isolation characteristic can be improved.
In the present invention, movable element main body serving as a portion of the movable element except for projections has a width serving as a length in a direction parallel to the widthwise direction of the distributed constant lines to be smaller than a width of each of the distributed constant lines, and a portion of the overlap portion of the movable element except for two ends in the movable element is notched. With this structure, even if the positioning error occurs in the widthwise direction of the movable element, all the projections can oppose the distributed constant lines, thereby suppressing the degradation of an ON reflection characteristic of the micromachine switch in that case.
In the present invention, a portion of the movable element having the projections is formed by notching two ends of the overlap portion of the movable element such that a width serving as a length in a direction parallel to the widthwise direction of the distributed constant lines is smaller than a width of each of the distributed constant lines. With this structure, the width of the portion of the movable element having the projections is smaller than that of the distributed constant line, thereby obtaining the same effect as in the above invention.
In this case, the width of the movable element main body serving as a portion of the movable element except for the projections may be equal to the width of the distributed constant lines. With this structure, the characteristic impedance on a gap becomes almost equal to that of each of the distributed constant lines. Thus, the degradation of an ON reflection characteristic of the micromachine switch can be prevented and an OFF isolation characteristic can be improved.
In the present invention, each of the projections has a rectangular shape. Thus, even if the positioning error occurs in the longitudinal direction of the movable element, the opposing area between the movable element and distributed constant lines is a predetermined area. Accordingly, a desired isolation characteristic can be obtained even in the above case.
In the present invention, a length, serving as a width of each of the projections, in a direction parallel to the widthwise direction of the distributed constant lines near the movable element main body serving as a portion of the movable element except for the projection is made larger than that away from the movable element main body. This increases a mechanical strength of the projections.
In the present invention, the movable element has a connection portion for connecting distal ends of the projections to each other. Thus, all the projections are simultaneously brought into contact with the distributed constant lines in an ON state of the micromachine switch, thereby improving an ON reflection characteristic.
In the present invention, at least one distributed constant line opposing the projections of the movable element does not oppose a movable element main body serving as a portion of the movable element except for the projections. That is, only the distal end portions of the projections of the movable element oppose the distributed constant lines. This greatly reduces the opposing area between the movable element and distributed constant lines, thereby obtaining a good OFF isolation characteristic.
In the present invention, at least one distributed constant line opposing the projections of the movable element also opposes a movable element main body serving as a portion of the movable element except for the projection. That is, the projections of the movable element and a part of the movable element main body oppose the distributed constant lines. Thus, a discontinuous portion of the micromachine switch in an ON state is only a portion where the movable element is in contact with the distributed constant lines, thereby obtaining a good OFF reflection characteristic.
Also, the present invention comprises at least two distributed constant lines disposed close to each other, a movable element arranged above the distributed constant lines such that distal end portions of the movable element oppose the distributed constant lines, respectively, and including a conductor, and driving means for displacing the movable element by an electrostatic force to bring the movable element into contact with the distributed constant lines, wherein at least one distributed constant line has at least two projections formed by notching an overlap portion of at least one distributed constant line, and the projections oppose the movable element. This can suppress the degradation of an ON reflection characteristic of the micromachine switch and improve an OFF isolation characteristic.
In the present invention, a width of the movable element serving as a length in a direction parallel to the widthwise direction of the distributed constant lines is equal to a width of a distributed constant line main body serving as a portion of at least one distributed constant line except for the projections, and at least one distributed constant line having the projections has a notched portion of the overlap portion of at least one distributed constant line except for two ends. With this structure, the characteristic impedance on a gap becomes almost equal to that of each of the distributed constant lines. Thus, the degradation of an ON reflection characteristic of the micromachine switch can be prevented and an OFF isolation characteristic can be improved.
In the present invention, a width of the movable element serving as a length in a direction parallel to the widthwise direction of the distributed constant lines is larger than a width of a distributed constant line main body serving as a portion of at least one distributed constant line except for the projections, and at least one distributed constant line having the projections has a notched portion of the overlap portion of at least one distributed constant line except for two ends. With this structure, even if the positioning error occurs in the widthwise direction of the movable element, all the projections can oppose the movable element, thereby suppressing the degradation of an ON reflection characteristic of the micromachine switch in that case.
In the present invention, a portion of at least one distributed constant line having the projections is formed by notching two ends of the overlap portion of at least one distributed constant line on the movable element side such that a width of a portion at which the projections are formed is smaller than a length in a direction parallel to the widthwise direction of the distributed constant lines. With this structure, the width of the portion of at least one distributed constant line having the projections is smaller than that of the movable element, thereby obtaining the same effect as in the above invention.
In this case, the width of the movable element may be equal to the width of a distributed constant line main body serving as a portion of at least one distributed constant line except for the projections. With this structure, the characteristic impedance on a gap becomes almost equal to that of each of the distributed constant lines. Thus, the degradation of an ON reflection characteristic of the micromachine switch can be prevented and an OFF isolation characteristic can be improved.
In the present invention, each of the projections has a rectangular shape. Thus, even if the positioning error occurs in the longitudinal direction of the movable element, the opposing area between the movable element and distributed constant lines is a predetermined area. Accordingly, a desired isolation characteristic can be obtained even in the above case.
In the present invention, the movable element does not oppose a distributed constant line main body serving as a portion, except for the projections, of at least one distributed constant line having the projections. That is, only the distal end portions of the projections of at least one distributed constant line oppose the movable element. Thus, a good OFF isolation characteristic can be obtained.
In the present invention, the movable element also opposes a part of a distributed constant line main body, which serves as a portion except for the projection of at least one distributed constant line having the projections. That is, the projections and a part of at least one distributed constant line main body oppose the movable element. Thus, a good OFF reflection characteristic can be obtained.
In addition, the present invention comprises at least two distributed constant lines disposed close to each other, a movable element arranged above the distributed constant lines such that distal end portions of the movable element oppose the distributed constant lines, respectively, and connecting the distributed constant lines to each other in a high-frequency manner upon contacting the distributed constant lines, and driving means for displacing the movable element by an electrostatic force to bring the movable element into contact with the distributed constant lines, wherein at least one distributed constant line has at least two first projections formed by notching an overlap portion of at least one distributed constant line, and the movable element has at least two second projections so formed as to oppose the first projections of at least one distributed constant line by notching an overlap portion of the movable element. This can suppress the degradation of an ON reflection characteristic of the micromachine switch and improve an OFF isolation characteristic.
In the present invention, at least an entire lower surface of the movable element is made of a conductor.
In the present invention, the movable element is made of a conductive member, and an insulating thin film formed on an entire lower surface of the conductive member.
In the present invention, the driving means comprises an electrode which is disposed apart between the distributed constant lines to oppose the movable element and to which a driving voltage is selectively applied.
In the present invention, the switch further comprises support means for supporting the movable element, the driving means is made of an upper electrode attached to the support means, and a lower electrode disposed under the upper electrode and opposing the upper electrode, and a driving voltage is selectively applied to at least one of the upper and lower electrodes.